Pulse-chopped electron beam source



'Dec.2,1969 H-LEBOUTEiF 3,482,139

PULSE-CHOPPED ELECTRON BEAM SOURCE Fil ed March 27, 1967 FIG 6 FIG 4 FIG :10

FlGrS ELECTRON SOURCE United States Patent O int. or. 1 101;; /26

US. Cl. 3155.25 15 Claims ABSTRACT OF THE DISCLOSURE A device for converting a continuous electron beam into a pulse-chopped beam, wherein a continuous beam is injected on the axis of a cavity in which it propagates under combined action of an axial magnetic field and a transverse UHF field so as to move along a divergent helical path until it strikes a terminal wall in which is cut a slot by which the beam is chopped into discontinuous pulses of electrons. These pulses emerge through this slot into a second cavity in which are also established an axial magnetic field and a transverse UHF field, and the size of said second cavity and magnitude of said fields is such that the pulses move along a convergent helical path until they are delivered through an aperture in a terminal wall of the second cavity along the common axis of both cavities.

BACKGROUND OF THE INVENTION This invention relates to a device for converting a continuous electron beam into a pulse-chopped beam.

A device of this kind, known in the prior art, is described in the French Patent No. 1,024,850 and provides a pulse-chopped electron beam from a continuous beam injected along the axis of a cavity in which there exists a uniform axial magnetic field and a transverse high frequency electric field. The high frequency therein is suitably chosen with respect to the magnetic field so that the electrons are caused to rotate at the cyclotron frequency and describe a helical trajectory whose radius progres sively increases as the electrons propagate forward in the axial direction.

At the extremity of the cavity there is disposed a terminal wall or plate which intercepts the electron beam so that the electrons move on the surface of the plate along a closed trajectory which may be circular or elliptical. The plate is provided with one or more radial slots which intersect the closed electron trajectory and permit the passage therethrough of a pulse-chopped electron beam, while the electrons impinging onto the plate outside the slots are absorbed therein. In this Way there are obtained electron pulses emerging from the cavity and distributed on a cylindrical surface having the same axis as the initial continuous beam, but in such a device no pulse emerges along the axis itself.

Thus, while the known provides a pulse-chopped beam at the output thereof, the resulting pulses are displaced from the axis of the device. Such displacement is clearly disadvantageous, especially where the pulse-chopped beam is to be used for example, in conjunction with an accelerator device requiring an injection along a particular trajectory with consistent accuracy.

BRIEF DESCRIPTION OF THE INVENTION The present invention consists of a device for converting a continuous electron beam into a pulse-chopped beam, comprising a first cavity in which said continuous beam is propagated along a divergent helical path about the axis of the cavity under the combined action of an axial magnetic field and a transverse high frequency elec- 3,482,139 Patented Dec. 2, 1969 tric field, the cavity having a terminal wall provided with one or more apertures through which discontinuous groups of electrons emerge from the cavity at a distance from the cavity axis, wherein the discontinuous electron groups are caused to propagate along a convergent helical path in a second cavity, adjacent the first one, under the combined action of an axial magnetic field and a trans verse high frequency electric field similar to those prevailing in the first cavity, said second cavity having a terminal wall provided with an aperture aligned with the cavity axis for the outlet of the beam.

Thus, the primary object of the present invention is to provide a device for converting a continuous electron beam into a pulse-chopped beam which entirely eliminates, or otherwise materially avoids, the disadvantages inherent in known devices of a similar nature.

It is an object of the present invention to provide an improved device for converting a continuous electron beam into a pulse-chopped beam which enables the electron pulses in the above described apparatus to be reset in the direction of the axis of the initial beam, so that the chopped beam may have substantially the same geometrical shape as the initial continuous beam, except for being chopped into discontinuous pulses.

Another object of the invention is to provide in the device slots of an improved shape in the terminal plate of the cavity in order to obtain certain special effects.

A further object of the invention is to provide the terminal plate of the cavity with a slot of a shape that enables alternatively either a chopped or a continuous beam to be obtained.

These and other objects, features and advantages of the present invention will become more apparent from the following detailed description thereof when taken in conjunction with the accompanying drawings, which set forth several embodiments of the invention, and wherein:

FIGURE 1 is a longitudinal section through a device in accordance with the invention;

FIGURE 2 is a cross-sectinon through the device of FIGURE 1;

FIGURES 3 and 4 represent two different embodiments of the slot 7 for use in the device of FIGURES 1 and 2;

FIGURE 5 shows an alternative embodiment in which the terminal partition plate 6 of the device of FIGURES 1 and 2 is replaced by a baflle;

FIGURES 6 and 7 show two different modifications to the battle of FIGURE 5;

FIGURE 8 shows a modification of FIGURE 2;

FIGURE 9 shows a modification of FIGURE 3;

FIGURE 10 shows a modification of FIGURE 4; and

FIGURE 11 shows a modification of FIGURE 5.

SPECIFIC DESCRIPTION OF THE INVENTION In carrying the invention into effect according to one embodiment by way of example, FIGURE 1 shows a device in which a continuous electron beam 1, issuing from an electron gun 2, enters with a speed v into a cavity 3, in which there is established by any conventional means (not shown) a uniform axial magnetic field of intensity B, while a high frequency source 4 of frequency f establishes in the same cavity a mode having a transverse high frequency electric component. Under these conditions the electrons propagate in cavity 3 along helical trajectories describing spirals of a progressively increasing diameter so that the spirals are contained within a cone 5. The cyclotron frequency F of the electron rotation is related to the field intensity B by the formula:

F being in megacycles/sec. if B is expressed in gauss;

3 M is the relativistic mass of the electrons and M their mass at rest.

At a distance I from the inlet, the cavity 3 is closed by a partition plate 6, provided with at least one radial slot 7, having, for example, the sectoral shape represented in FIGURE 2.

In this device, the electrons arriving at the plate 6 sweep this plate along a circle 8 whose diameter depends upon the values of the field B, the length l, the energy supplied by source 4, and the initial velocity v. The slot 7 is disposed so as to interrupt the circle 8. Thus it is evident that at the output of slot 7 there is obtained a series of pulses 9 each having an electrical length equal to the sectoral angle of slot 7. However, these pulses emerge from slot 7 outside of, i.e., displaced from, the axis of cavity 3.

In order to reset the pulses along the axis and reestablish the geometrical shape of the initial inlet beam 1, a second cavity 10 is disposed adjacent the first cavity 3. The second cavity is given the same geometrical dimensions as the first one, and there are established in cavity 10 the same axial magnetic field l3 and the same transverse high frequency electric field component as in cavity 3, use being made preferably, but not necessarily, of the same high frequency source 4 providing frequency f. With certain parameters, which will be specified here inafter, the bunches 9, which describe around the lines of force of the field B spirals 11 of a progressively decreasing diameter, emerge through the orifice 12 in the terminal plate 12' of cavity 10 and form on the common axis of cavities 3 and 10 a pulse-chopped beam having the same shape and the same diameter as the continuous input beam 1. This pulse-chopped beam may therefore be utilized in an accelerator.

For the parameters of the system, the following rules apply:

Having chosen 1 and B and deriving F from Equation 1, the frequency f of source 4 is chosen from the values that satisfy either of the following two equations:

F 1 I 2n+1 (3) In these equations, 12 is a positive integer, and m is either a positive integer or zero.

Equation 2 is used when it is desired to obtain a phase shift 1r between homologous points of the two cavities; then the numbers In and n are given values such that F/ be positive (condition 2m n).

Equation 3 is utilized when it is desired to have a zero phase shift at homologous points of the two cavities.

The source of frequency f excites in the cavity a wave whose wave length when propagating in an infinite wave guide having the same external sizes as the cavity is Ag. This wave length is related to the length l of the cavity by the relation M1 2 where q is a certain integer. This number can consequently be determined from Equation 4 after kg has been measured.

Finally, the initial speed v is determined by the relation l f' 1 where q is the integer number given by Equation 4, and n' =n when using Equation 2, or n'=2m+1 when using Equation 3.

The radial position and the length of slot 7 are chosen in such a manner that this slot intersects the circle 8 of radius r on the partition plate 6, which circle is swept d by the beam. The radius 4 depends on the field B and on the length l as well as on the frequency f and the energy of source 4. Thus the radius r may be varied, if desired, by varying the energy.

FIGURES 3, 4, 5 represent some possible embodiments providing a different shape slot 7 than provided in FIGURE 2. In FIGURE 3, slot 7 has the form of a V having its opening towards the axis of the device. The use of this shape is to permit a reduction of the beam current when the energy of source 4 increases. Indeed, as a result of an increase of radius r, due to an increase of energy, the slot width swept by the beam decreases and consequently the chopped pulses become shorter and the mean current is reduced.

FIGURE 4 is distinguished from FIGURE 3 by the fact that the sides of the V are curvilinear. With this disposition, the phase of the pulses is shifted in one or the other direction depending on the sense of rotation of the beam on circle 8. This phase variation follows a desired law, determined by the shape of the curvilinear sides of the slot, and at the same time the length of the chopped pulse varies with the energy applied.

In FIGURE 5 the partition plate 6 of FIGURES 1 and 2 is replaced by a baffie 13 whose edge 14 extends in the plane passing through the axis of the cavity which is supposed here to be rectangular. This case may be considered as that of slot 7 of FIGURE 2 in which the sector angle has been increased to Then the beam sweeping the circle 8 is chopped into pulses each of which has a duration equal to the interval between two consecutive pulses. The battle, replacing the partition plate, is advantageous when it is desired to achieve a strong coupling between the cavities 3 and 10. It is, of course, possible to make use of other sector angles, lower or higher than 180, simply by making the edge 14 in the form of a V, inverted or not, as shown in FIGURES 6 and 7 wherein the apex 15 of the V is situated on the axis of the cavity.

FIGURES 8 through 11 show modifications of the embodiments of FIGURES 2 to 5, respectively. In these modifications the radial slot 7 of FIGURES 2, 3 or 4, or the baflle 13 of FIGURE 5, are combined with a small axial hole 16 of radius r sufficient to permit the passage of an axial beam having substantially the same diameter as the inlet beam 1. This combination provides, in particular, in FIGURE 8 the familiar key-hole shape, while in FIGURE 11 a simple semi-circular notch is pro- 'vided in bafile 13. This disposition makes it possible to obtain, at the outlet of the hole 12. in FIGURE 1, alterna tively, either a pulse-chopped beam such as described, or a continuous beam representing the beam 1 after propagation along the axis of the device without any deflection. Switching in this way from one mode to another can be achieved simply by removing or resetting the field of source 4.

Various modifications may be made to the embodiments herein described and illustrated by way of example. For instance, it is not necessary that cavity 10 should be supplied by the same high frequency source as cavity 3. For example, two different sources may be employed with slightly different frequencies. Under such conditions the phase of a pulse 9 in cavity 10 slowly shifts with respect to the phase of the outlet at the slot 7. Then only a portion of pulse 9 arrives at the outlet 12 at the suitable phase for emerging and the remainder is lost in the terminal wall of cavity 10. This provides an additional means for shortening the pulse 9.

We have shown and described one embodiment in accordance with the present invention. It is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art and we, therefore, do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

We claim:

1. A device for converting a continuous electron beam into a pulse-chopped beam comprising a pair of adjacent cavity resonators separated by a wall provided with at least one aperture displaced from the common axis of said cavities, means for injecting along the axis of one of said cavities a continuous electron beam, means for establishing in both of said cavities a common axial magnetic field and a transverse high frequency electric field whereby said beam is propagated within said first cavity along a divergent helical path along said axis until said beam strikes said wall along a path of rotation, said aperture in said separating wall being disposed on said path of rotation so that said beam is chopped into discontinuous pulses of electrons while sweeping across said aperture, the size of said second cavity and magnitude of said fields therein being such that said discontinuous pulses emerging from said aperture into said second cavity propagate therein along a convergent helical path terminating on said common axis, and a terminal wall of said second cavity provided with an aperture aligned with said common axis for the outlet of said pulses.

2. A device as claimed in claim 1, wherein said first and second cavities have the same shape and geometrical dimensions.

3. A device as claimed in claim 1, comprising means for exciting in both cavities high frequency fields at the same frequency.

4. A device as claimed in claim 1, wherein said aperture in said separating wall between cavities is a slot in the form of a V opened toward said common axis.

5. A device as claimed in claim 4, wherein the sides of the V are rectilinear.

6. A device as claimed in claim 4, wherein the sides of the V are curvilinear.

: 7. A device as claimed in claim 1, wherein said separating wall is a baffie having one edge in the axial plane of the cavities.

8. A device as claimed in claim 1, wherein said separating wall is a baffie whose edge comprises a point at said common axis.

9. A device as claimed in claim 1, wherein said aperture in said separating wall is combined with a circular hole in registry with said common axis.

10. A device as claimed in claim 1, wherein said aperture in said separating wall is a sectoral slot combined with a circular hole in registry with said common axis, thereby forming a key-hole shaped aperture.

11. A device as claimed in claim 4, wherein said slot is combined with a circular hole in registry with said common axis.

12. A device as claimed in claim 7, wherein a semicircular aperture having its center of curvature on said common axis is cut in said edge.

13. A device as claimed in claim 8, wherein a substantially semi circular aperture having its center of curvature on said common axis is cut around said point of said edge.

"14. The device as claimed in claim 1, wherein the frequency f of said electric field relates to the cyclotron frequency F of the electron rotation by the relationship where n is a positive integer and m is either a positive integer or zero.

15. The device as claimed in claim 1 wherein the frequency f of said electric field relates to the cyclotron frequency F of the electron rotation by the relationship F 1 rmt where n is a positive integer.

References Cited UNITED STATES PATENTS 2,870,368 1/1959 Cuccia 3155.27 X

HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R. 315-3, 5 

